10 years later, Higgs Boson explorers publish refined measurements

Newswise – Particle physics changed forever on July 4, 2012. That was the day the two major physics experiments at CERN’s Large Hadron Collider (LHC), CMS and ATLAS, jointly announced the discovery of a particle that matched the properties of the Higgs galaxy. Particle: A particle that was theorized decades earlier. The discovery confirmed the latest piece in the Standard Model of particle physics.

Over the next decade, physicists at CMS and ATLAS have tenaciously studied the Higgs boson, investigating its properties and unlocking its secrets.

This week, on the tenth anniversary of the Higgs discovery, CMS and ATLAS have released extensive new measurements of this particle in a special edition of the magazine Nature† Both collaborations have measured the properties of the Higgs boson more accurately than ever, but neither has discovered any surprises so far.

“The Particle That Was Discovered” [in 2012] increasingly resembles the standard-model Higgs boson,” said Kétévi Assamagan, an ATLAS physicist at the U.S. Department of Energy’s Brookhaven National Laboratory, who chaired the experiment’s Higgs group from 2008-2010. “Yet there is room for new physics.”

“What’s really impressive is how well the experiments measured these properties,” said Sally Dawson, a theorist at Brookhaven and co-author of the book. The Higgs Hunter’s Guide† “We never guessed… It’s truly phenomenal.”

“Now we know a lot about the Higgs because particle physics predicted how the Higgs would be produced, how it would decay, the signatures that we would see. And it looks like it’s going exactly as predicted.”

Liza Brost is an ATLAS physicist who has been studying the Higgs boson since its discovery — literally. (She started working at CERN on July 3, 2012.) “It’s really nice to have this new particle that we can analyze in detail and see what it is, how it behaves,” said Brost, who now works for Brookhaven. works.

Daniel Guerrero, a CMS researcher at DOE’s Fermi National Accelerator Laboratory, agrees: “After we discovered the Higgs boson, a completely new field of research opened up for experiments at the LHC.”

In the past 10 years, the LHC completed its second set of data collection, called Run 2. During this time, the collision energy was increased from 8 tera-electron volts (TeV) to 13 TeV, increasing the rate of Higgs production and resulting in much more data for the experiments to collect and analyze.

For example, ATLAS estimates that about 9 million Higgs bosons were produced in the ATLAS detector during run 2-30 times more than in 2012 (although they analyze only a fraction of that number).

“The Higgs boson just got ‘bigger’ [in the last decade]”, says Fermilab CMS researcher Nicholas Smith, metaphorically speaking (the mass of the Higgs boson remains about 125 GeV, now measured with a precision of 0.1%). “We’ve only gotten better and better at seeing it.”

Precision measurements

Higgs bosons are created by accelerating beams of protons around the LHC’s 17-mile circular tunnel at nearly the speed of light and causing them to collide. Two beams travel in opposite directions and collide at four points along the ring, including at the CMS and ATLAS detectors. The collisions cause the formation of new particles, which sometimes interact and turn into a Higgs boson.

Studying the combinations of particles that can make a Higgs particle— called production channels or modes — and the particles into which it decays — called decay modes — give physicists a better understanding of the particle.

The new CMS and ATLAS results were obtained by combining several separate analyzes of Higgs bosons production modes and their corresponding decay modes.

“The combination actually takes over the division of labor [by separate analyses] and then recombining it into something interpretable as one physical result,” said Smith, co-founder of the CMS Higgs combination group. “More [decay modes] you can cover, the stricter limits you can put on how Higgs production behaves, and vice versa: the more production modes you watch, the more you can put strict limits on how it decays.”

Some of the key measurements include how the Higgs interact with other particles. These interactions, or couplings, are part of the mechanism by which it gives Higgs mass to other fundamental particles.

The ATLAS collaboration has measured the Higgs couplings with the top quark, bottom quark and tau lepton with uncertainties ranging from about 7% to 12%, as well as the couplings with the W and Z bosons with uncertainties of about 5%. .

Many of the individual Higgs properties listed in the CMS document were measured to an accuracy of better than 10% with a combined precision of almost 5% – a big improvement over the combined precision of more than 20% in 2012.

All new CMS and ATLAS measurements were consistent with the Standard Model predictions within uncertainties. But that doesn’t mean there aren’t new physics to be found, Guerrero says. “The effects of physics beyond the Standard Model may still be hidden in those uncertainties, so that’s not a showstopper,” he says. “Actually, we want to go further and see if, after we go to higher precision, we can start seeing anomalies.”

Room for discovery

Indeed, there is much room for new phenomena beyond the Standard Model, as some of the key properties of the Higgs boson have yet to be measured by both CMS and ATLAS. These include some of its rare decay modes and the Higgs boson’s coupling to itself.

This self-coupling of the Higgs boson is a phenomenon that has been intensively studied by CMS and ATLAS, both of whom impose limitations on it in their new papers. It also has to do with Production of Higgs pair, an extremely rare interaction where two Higgs bosons, instead of just one, are created in a single production channel; CMS and ATLAS have not yet observed it, but have set new limits on the likelihood of this process occurring.

Ultimately, observing Higgs self-coupling and Higgs pair production will allow physicists to better understand a property called the Higgs potential. This property of the field generated by the Higgs “is related to the asymmetry between matter and antimatter in the early universe and the breaking of electroweak symmetry — all these huge questions that we usually don’t even get close in our industry,” Bros says. .

Now, however, physicists are looking forward to the data collected during LHC run 3 , which began on July 5 and will last nearly four years. They all agree that the next steps for Higgs research require more data — and a lot.

With more data, the experimental measurements could become even more accurate, and that will prompt theorists to calculate their predictions more accurately, too, Dawson says. One day, if a more accurate prediction diverges from the experimental measurements of the Higgs boson, it could indicate new physics.

“The discovery of the Higgs gave us some direction,” Assamagan says. “We have also devised better techniques for analyzing the process [and] we have improved our understanding of the detectors. So much progress has been made. And I think we are definitely in a better position now to discover new physics if there is one.”

This research was supported by the DOE Office of Science.

Fermilab is America’s premier national laboratory for particle physics research. Fermilab, a US Department of Energy Office of Science lab, is located near Chicago, Illinois, and is operated by the Fermi Research Alliance LLC. Visit the Fermilab website at: https://www.fnal.gov and follow us on Twitter @Fermilab

Brookhaven National Laboratory is supported by the Office of Science of the United States Department of Energy.

The DOE Office of Science is the largest proponent of basic science research in the United States, working to address some of the most pressing challenges of our time. For more information visit https://science.energy.gov

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